While heat exchangers formed from bundles of tubes, thermosyphonic heat exchangers, and the utilization of heat exchangers in hydrocarbon production is not per se new, none are believed to teach or suggest the concepts embodied in the present invention.
In hydrocarbon production, heat exchangers have been employed in some capacity to heat up recovered fluids in cold areas such as Alaska or the like, to facilitate better flow or prevent the formation of hydrates, ice, or other matter within the pipeline. Prior art teachings further include, as further discussed infra, so-called keel coolers as employed in vessels and offshore platforms, which may include a bundle of three or more tubes which may directly engage a cooling body of water, such as an ocean or the like; in a vessel, the keel cooler may be located adjacent to the keel, exterior of the vessel, hence the name.
Recent advances in geophysical exploration methods have located deeper oil and gas reservoirs. Production from these reservoirs is significantly hotter than shallower production. The hotter production must be cooled before it can be economically pipelined or processed. For transport by subsea pipeline, the production must be cooled to 150-160 degrees F. or expensive materials and special designs will be required. For gas processing, separation, sweetening, and dehydration--the gas is normally cooled to 120-130 degrees F. or less. The gas cannot be effectively processed at a higher temperature. In addition, chlorides associated with aqueous phase attacks stainless steel at temperatures above 130-135 degrees F., so there exists a universal need for an economical means to cool hydrocarbon production on an offshore platform.
From a satellite facility, where offshore gas and oil wells are produced, the produced fluids are usually pipelined to a Central Processing Facility (CPF). The hot well fluids must be cooled prior to entering the pipeline. A high temperature fluid passing through the pipeline causes extensive pitting and metal loss, with alloy steel or nickel alloys particularly, at the waterline. One possible solution is to construct the hot upstream section of the pipeline with a double wall system, which will keep the outside of the pipe from getting too hot. This type of construction, however, is expensive, and is estimated to cost approximately twice as much as a single wall pipeline.
Alternatively, the gas may be cooled on the satellite platform before introduction to the pipeline. Conventional cooling methods include fin-fan coolers or seawater cooling via traditional heat exchangers. One method in common use is the fin-fan cooler, in which the production is directed to a large array or bank of finned tubes. Air is blown across the tubes with a motor driven blower to cool the tubes. Fin-fan coolers are usually quite large, heavy, and installed on the top deck of the platform, where space is at a premium, to obtain good cooling efficiency. The fan motor is usually driven by electricity, thereby requiring a power supply at the platform. Electricity is either generated by an on-site gas-driven turbine/generator set, or can be transmitted from a nearby platform with a subsea cable. An offshore turbine generator installation is generally large, complex, and expensive. It requires a large deck space and a continuous supply of clean natural gas. The clean gas must be pipelined from the CPF or other facility where a sufficient supply of clean gas is available.
For a 50 million SCFD gas flow rate produced at 300 degrees F., a system for cooling to a temperature of 160 degrees F. would consist of a 13 MM (million) Btu/hr fin-fan cooler with two (2) 50 HP electric motors. The cooler requires a 15.times.30 foot area of deck space, and would use about 850,000 Btu/hr of energy as natural gas to drive the fans. At a gas price of $2.50/Million Btu, this results in an annual cost of $18,500. The capital cost for the cooler, generator set, fuel gas line, motor controls, and platform is estimated to exceed $2,000,000.00.
The below patents are cited as having at least cursory pertinence to the concepts enunciated in the present invention:
______________________________________ Patent Number Inventor Date of Issue ______________________________________ 5573060 Adderley et al 11/12/1996 4924936 Mckown 05/15/1990 4043289 Walter 08/23/1977 4040476 Telle et al 08/09/1977 3648767 Balch 03/14/1972 3472314 Balch 10/14/1969 2193309 Wheless 03/12/1940 1913573 Turner 06/13/1933 735449 Berger 08/04/1903 ______________________________________
U.S. Pat. Nos. 3,648,767 and 3,473,314 teach thermosyphonic systems to facilitate circulation of fluid to be thermally affected in a heat exchanger.
U.S. Pat. No. 735,449 teach a jacketed heat exchangers to affect temperature on hydrocarbons recovered from a well. U.S. Pat. No. 2,193,309 teaches a heat exchanger system for warming high pressure gas wells, so as to the prevent formation of snow or ice particles or the like; other systems taught heating of gas to prevent formation of hydrates.
Heat exchangers incorporating an array of tubes to form a bundle configuration is taught to some extent in U.S. Pat. No. 1,913,573 for a Radiator dated 1933. Note also U.S. Pat. No. 4,040,476 for a "Keel Cooler with Spiral Fluted Tubes", which system is submerged in a marine environment to cool the fluid therein. See also U.S. Pat. No. 4,043,289 which contemplates a keel cooler including a tube bundle. U.S. Pat. No. 5,573,060 is another example of heat exchangers directly employed in seawater.
Geothermal heat exchange systems may employ exchange from one medium to another, including cold depths of a water body to warmer shallows of the same body, although none are believed to contemplate the apparatus or methodology of the present invention.
Lastly, U.S. Pat. No. 4,934,936 teaches a "Multiple, parallel packed column vaporizer" that contemplates a bundle of tubes jacketed in an enclosure for heat exchange.
While heat exchangers have been employed in seawater, some discussed above, there exists a significant problem in deploying high pressure, high temperature heat exchangers directly in salt water, because any direct contact of the metal forming the heat exchanger with sea water may cause same to boil, facilitating tremendous corrosion and/or pitting problems for most metals, ferrous and non-ferrous; most grades of stainless steel and aluminum are not immune to this problem.
Recent advances in hydrocarbon recovery techniques have resulted in successful wells in high depth reserves deep offshore. Recovery of gasses from these areas has facilitated new problems heretofore unexperienced in the industry. For example, natural gas from deep reserves exits the production platform at both a high temperature and high pressure, presenting problems associated with containment as well as corrosion of the system due to the salt water environment, as discussed supra.
Because standard pipelines cannot handle the pressure and high corrosion, there has been some discussion of employing expensive titanium pipelines, but the cost would be generally cost prohibitive and dangerous to maintain long high pressure system. Chokes or the like may be employed to reduce the pressure to some extent, but the real answer in facilitating satisfactory production is to reduce the temperature of the stream, which will allow the use of conventional pipelines and cost effective treatment facilities. As may be discerned by a review of the above, the known prior art has failed to contemplate such a system.